This invention relates to proximity instruments and, more particularly, to a calibration circuit for a fiber optic proximity instrument having automatic adjustments for changes in the intensity of the reflected light.
Fiber optic proximity probes are devices which use a light beam transmitted from a light source by way of a bundle of light conducting optical fibers from a light source to a target. The light is reflected off the target and returned along other optical fibers in the bundle back to a light intensity sensor in the instrument. The intensity of the returning light is a function of the distance between the proximity probe tip and the target, so the output of the light sensor can be a very precise measure of that distance, or more importantly, changes in that distance.
Because of variations in certain operating parameters such as surface reflectivity of the target, light intensity output of the light source, transmission efficiency of the fiber optic bundle, and other factors which influence the intensity of the light transmitted, reflected, returned and sensed, it is necessary to readjust the instrument if any one of these factors are changed during operation. Although this readjustment is a simple and fast procedure, and is cause for no concern in most situations, there are certain applications in which the need to readjust the instrument for each use is an unacceptable requirement, for example, where the instrument is in an inaccessible location or where its readings must be available continuously or at unpredictable intervals. In certain of these installations, the instrument probe is permanently mounted relative to a target and the mean gap between probe and target is constant. In such permanent situations it would be necessary to provide correction for errors due to degradation of the optical light path transmission efficiency caused by such factors as optical fiber breakage, debris on the face of the transducer, and variations in the surface reflectivity of the target.